Vorticity modeling is used to simulate the flow around a surface-mounted prism. The objective is to examine whether vorticity modeling can give satisfactory information about surface pressure fluctuations which are mostly due to the outer or inviscid flow. Differences between results obtained with vorticity modeling and what one should expect from DNS and LES are pointed out. These include the difference between the governing equations, the shortcomings of having a 2-D simulation and the realization of introducing and convecting vorticity to simulate some turbulence aspects. All necessary details needed for the setup of vorticity modeling for complex flows, such as the one considered here are given. These details include choice of elements, the calculation of velocities, the application of boundary conditions and calculation of pressure. The numerical procedure and our use of parallelization in the code are explained. The results presented on velocity magnitude, vorticity and pressure show important characteristics of the flow field in terms of interaction of positive and negative vorticities and their effects on the surface pressure. The calculated peak and mean values for the pressure coefficients at the leading edge are close to those measured in separating flows over prisms. / Master of Science

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24892号 / 工博第5172号 / 新制||工||1988(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 渦岡 良介, 教授 安原 英明, 教授 肥後 陽介 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Prestressed Concrete Cylinder Pipe (PCCP) is a large-diameter and high-pressure conduit for drinking water and wastewater transmission. Due to its large diameter, high pressure, and mode of breakdown, PCCP failures usually have catastrophic consequences. To mitigate failures, it is very important to assess the condition of the pipe and take proactive measures, such as repair, rehabilitation, or replacement. There are many challenges in assessing the condition of PCCP. PCCP has a complex structure with several layers of materials (e.g. mortar coating, prestressing wire, steel cylinder, and concrete core) working together under loading. This means that there are many factors that can cause pipe failure, and that failure mechanisms are complicated. Data collection could be difficult, and existing data are often unavailable or unreliable. Considerable research has been conducted by scholars and engineers in developing models to evaluate the condition of PCCP. There are mainly two types of models: statistical models, and numerical models using finite element method. Statistical models consider only a few factors, such as pipe age and failure rate, to predict the failure of PCCP. However, the failure of PCCP can be caused by many other factors including pipe material, and loading conditions. Models only considering a few factors are not robust enough for reliable results. The current numerical models assume that all broken wires are centrally distributed in the same location and broken wires have no prestress, thus all broken wires are completely removed from the model. These assumptions could be overly conservative when actual broken wires are distributed in different locations along the pipeline and broken wires have remaining prestress due to the bond between the wire and mortar coating. Therefore, a more comprehensive numerical model is needed to have a better understanding of the condition of PCCP. In this research, an extensive literature and practice review was conducted on PCCP failures to understand the critical factors that affect pipe condition. The available technologies commonly used to detect pipe defects were reviewed in order to better understand the accuracy and uncertainties of the collected data. Existing models were reviewed to better understand their limitations and to advance the research on condition analysis of PCCP using numerical models. Based on these comprehensive reviews, this dissertation proposed a numerical model to analyze the condition of PCCP for its long-term performance management. Detailed structural components such as concrete cores, prestressing wires, steel cylinder, and mortar coating were modelled. The interactions between different layers of pipe components were considered. An algorithm was proposed to account for the bond between the prestressing wire and mortar coating, which is a critical factor for the condition of PCCP with broken wires. A FORTRAN program was developed to assign linear stress distribution between the broken point and the full-prestress resuming point. The proposed numerical model was verified utilizing data from lab tests and forensic study. Lab test data helped to understand the functionality of the model and to verify the model parameters used in analyzing pipe components and the simulation of interactions between different layers. The forensic data helped to verify the model under actual field working conditions of the pipe. Validation of the proposed numerical model was conducted using a 66-inch Embedded Cylinder Pipe and two Lined Cylinder Pipes (42-inch and 48-inch, respectively) from a water utility. In the validation, field data were collected for model development. The simulation results were consistent with the field observation, which proved the validity and applicability of the proposed numerical model in practice. A series of sensitivity studies were conducted to investigate the impact of longitudinal and circumferential location on the structural integrity of the pipe. These investigations showed that considering the actual longitudinal and circumferential location of broken wires is very important to get accurate analysis of pipe condition, while assuming that all broken wires fail in one longitudinal location (assumptions by current numerical models for PCCP) will overestimate the actual damage to the pipe caused by broken wires. To consider the bedding condition, a critical factor for PCCP, the four most common bedding types found in practice were analyzed. Results show that poor bedding could lead to cracks in PCCP, which could cause corrosion in prestressing wires. Therefore, it is very important to account for bedding conditions in the PCCP analysis. The model presented in this dissertation is more comprehensive and robust compared with existing numerical models, and could provide a better understanding of the condition of PCCP. This is because the proposed model considers the contribution of remaining prestress in broken wires due to the bond between the wire and mortar coating. This model can consider the actual longitudinal and circumferential location of broken wires rather than centrally distribute them, and it can consider the actual bedding locations, and the interaction between different layers of materials. This model was calibrated using lab test data and forensic data, and was further validated using field data which showed consistence between simulation results and field observations. The proposed model does have limitations due to limited availability of data and assumptions. Material tests were not conducted to verify the material properties used in the model, which could cause accuracy issues in the results. A full-scale simulation of the interaction between prestressing wire and mortar coating was not considered because it could significantly increase the computation time. Lab tests were not conducted to verify the parameters used for the simulation of interaction between concrete core and steel cylinder which could lead to accuracy problems. Finally, it is acknowledged that the model was only validated in one water utility and validations in more geographically distributed utilities might further test the model's validity and robustness. Nonetheless, the comprehensiveness and robustness of this proposed model improved the analysis of the condition of PCCP. The findings and results of this research will provide guidance for better management of PCCP pipelines for water utilities, and provide reference for future research on numerical modeling of PCCP as well. / Ph. D.

No / This paper proposes that the changes which occur in composition of the bed load during the transport of mixed-grain-size sediments are largely controlled by the distributions of critical entrainment shear stress for the various size fractions. This hypothesis is examined for a unimodal sediment mixture by calculating these distributions with a discrete particle model and using them in a probabilistic calculation of bed-load composition. The estimates of bed-load composition compare favorably with observations of fractional transport rates made in a laboratory flume for the same sediment, suggesting that the hypothesis is reasonable. The analysis provides additional insight, in terms of grain mechanics, into the processes that determine bed-load composition. These insights strongly suggest that better prediction methods will result from taking account of the variation of threshold within size fractions, something that most previous studies have neglected.

No / This paper presents the structural behaviour of cold-formed steel cassette wall panels subjected to in-plane shear loads. To understand the influence of configuration, lining material and connector arrangement on the overall shear behaviour of typical cassette wall panels, different lining materials, fastener spacing and positions, edge stiffeners and specific boundary conditions were assumed in the numerical simulations. The comparison and analysis presented in this paper demonstrate typical effect factors to the load-bearing capacity of selected wall panel systems. In particular, the effect of wall opening to the structural shear behaviour of wall panels is highlighted.

This thesis focuses on the direct numerical simulation of the combustion of a single aluminum droplet with phase change. For this purpose, the Volume of Fluid (VOF) method is employed for the direct numerical simulation of two distinct phases. To model the droplet combustion, the phase change (evaporation) and chemical reactions are modeled by setting source terms for each governing equation. This work proposes a new form of species source term by phase change, derived using the local instant formulation of two-phase flow. The Stefan problem is used to verify the modified source term. Evaporation fluxes calculated with both modified and conventional sources are compared, demonstrating that the modified species source term yielded mass flow rates closer to theoretical values, with an error rate of less than 20%. The instabilities of source terms in the droplet case are also analyzed, revealing that surface tension and chemical reactions cause numerical errors arising from the sharp discontinuities at the interfacial cells. The model’s validation includes a comparison with a benchmark case, assessing the temporal evolution of droplet diameter change and temperature fields. While the diameter change aligns reasonably with the benchmark, the temperature fields do not reach the benchmark’s flame temperature due to numerical diffusions. Furthermore, the molar fraction of aluminum gas at the interface closely matches experimental values, although the overall spatial distribution of molar fraction of species does not align with the benchmark.

Pressure Gain Combustion (PGC) systems have gained significant focus in recent years due to its potential for increased thermodynamic efficiency over a constant pressure cycle (or Brayton cycle). A rotating detonation combustor (RDC) is a type of PGC system, which is thermodynamically more efficient than the conventional gas turbine combustor. One of the main aspects of the detonation process is the rapid burning of the fuel-oxidizer mixture, due to which there is not enough time for the pressure to equilibrate. Therefore, the process is thermodynamically closer to a constant volume process, which is thermodynamically more efficient than a constant pressure cycle. RDC, if integrated successfully with a turbine, can increase thermal efficiency and reduce carbon emissions, especially when hydrogen is introduced into the fuel stream. However, due to highly unsteady flow generated from RDC, a systematic approach to transition the flow exiting the RDC to supply steady, subsonic flow at the turbine inlet has not been developed so far. Numerical simulations serve as a valuable tool to provide insight into the flow physics and to optimize the RDE design. Numerical studies have explored RDC by utilizing high-fidelity 3D simulations. However, these CFD studies require significant computational resources, due to the large differences in length and time scales between the flow field and the chemical reactions involved. The motivation of this dissertation is to investigate these research gaps and to develop computationally efficient methods for RDC designs to be integrated with downstream turbine section. First, this research work develops a methodology to predict the unsteady flow field exiting an RDC using 2D reacting simulations and to validate the approach using experimental measurements. Next, computational techniques are applied to condition the flow within the annulus by strategically constricting the flow area. A design of experiment (DoE) study is used to optimize the area profiling of the combustor. Additionally, the performance of the profiled design is compared against the baseline and the conventional nozzle design used in the literature. However, these numerical works use a perfectly premixed condition, whereas, the actual setup consists of discrete fuel/oxidizer injectors providing a non-uniform mixture in the combustor. To eliminate the assumption of perfectly premixed conditions, a method is developed to model the dynamic injector response of fuel/oxidizer plenums. The goal of this approach is to provide an inhomogeneous mixture composition without having to resolve/mesh the individual injectors. This research work provides a robust and computationally efficient methods for minimizing unsteadiness, maximizing pressure gain, and modeling dynamic injector response of an RDC. / Doctor of Philosophy / Traditional gas turbine combustor utilizes deflagration combustion. In recent years, detonation-based combustion has been explored as an alternative to enhance the efficiency of a modern gas turbine combustor. Rotating Detonation Combustor (RDC) utilizes detonation-based combustion and is thermodynamically efficient compared to conventional gas turbine combustors. The RDC consists of a detonation wave front and an oblique shock wave, which travel towards the exit of the combustor. Thus, the flow exiting the RDC is highly unsteady. The turbine requires a relatively steady flow at the inlet guide vanes. Therefore, the flow exiting the RDC needs to be conditioned before integrating with a downstream turbine section to gain the thermodynamic benefits of RDC. Numerical simulation of an RDC provides additional flexibility over experiments in understanding the flow physics. In addition, simulations are vital in optimizing the RDC designs such that the flow exiting the combustor is relatively uniform without comprising the pressure gain benefits of RDC. However, one of the challenges is that the RDC simulations are computationally expensive. Therefore, computationally efficient methods are required to understand and optimize the RDC designs to minimize the unsteady flow behavior and maximize the pressure gain. The objective is to utilize 2D and 3D reacting simulations to understand the flow behavior and to develop an optimization workflow to condition the flow exiting the combustor. Additionally, the optimized design is evaluated against the baseline and the conventional design used previously in the literature. Moreover, in most RDCs, the fuel and oxidizer are injected using discrete injectors. Due to the discrete injection, the fuel/oxidizer mixture is never perfectly premixed and results in a localized variation in fuel-oxidizer composition in the combustor. A novel method is developed to model the dynamic injector response of discrete fuel/oxidizer injection. The goal is to provide an inhomogeneous mixture composition without having to resolve/mesh the individual injectors. The emphasis of this study is to provide insight into the importance of flow conditioning exiting the RDC and the development of efficient CFD methods to optimize RDC to seamlessly integrate with a downstream turbine section.

A high resolution spectrally accurate three-dimensional flow solver is developed in order to simulate convection dominated fluid flows. The governing incompressible Navier Stokes equations along with the energy equation for temperature are discretized using a second-order accurate projection method which utilizes Adams Bashforth and Backward Differentiation formula for temporal discretization of the non-linear convective and linear viscous terms, respectively. Spatial discretization is performed using a Fourier/Chebyshev spectral method. Extensive tests on three-dimensional Taylor Couette flow are performed and it is shown that the method successfully captures the different states ranging from formation of Taylor vortices to wavy vortex regime. Next, the code is validated for convection dominated flows through a comprehensive comparison of the results for two dimensional Rayleigh Benard convection with the theoretical and experimental results from the literature. Finally, fully parallel simulations, with efficient utilization of computational resources and memory, are performed on a model three-dimensional axially homogeneous Rayleigh Benard convection problem in order to explore the high Rayleigh number flows and to test the scaling of global properties.

Doctor of Philosophy / Department of Biological & Agricultural Engineering / Ronaldo G. Maghirang / Emissions of particulate matter (PM) are an increasing concern for large open beef cattle feedlots. Research is needed to develop science-based information on PM emissions and abatement measures for mitigating those emissions. This research was conducted to (1) measure PM concentrations emitted from large cattle feedlots, (2) compare different samplers for measuring concentrations of PM with equivalent aerodynamic diameter of 10 µm or less (PM10), (3) evaluate the relative effectiveness of pen surface treatments in reducing PM10 emissions, and (4) predict PM control efficiency of vegetative barriers. Concentrations of PM with equivalent aerodynamic diameter of 2.5 µm or less (PM2.5), PM10, and total suspended particulates (TSP) upwind and downwind of two large cattle feedlots (KS1, KS2) in Kansas were measured with gravimetric samplers. The downwind and net concentrations generally decreased with increasing water content (WC) of the pen surface; for effective control of PM emissions from feedlots, it appears that pen surface WC should be at least 20% (wet basis). Three types of samplers for measuring PM10 concentrations in feedlots KS1 and KS2 were compared: Tapered Element Oscillating Microbalance™ (TEOM), high-volume (HV), and low-volume (LV) PM10 samplers. Measured PM10 concentration was generally largest with the TEOM PM10 sampler and smallest with the LV PM10 sampler. A laboratory apparatus was developed for measuring the PM10 emission potential of pen surfaces as affected by surface treatments. The apparatus was equipped with a simulated pen surface, mock cattle hooves that moved horizontally across the pen surface, and PM10 samplers that collected emitted PM10. Of the surface treatments evaluated, application of water (6.4 mm) and hay (723 g/m2) exhibited the greatest percentage reduction in PM10 emission potential (69% and 77%, respectively) compared with the untreated manure layer. Computational fluid dynamics (CFD) was applied to predict airflow and particle collection by a row of trees (2.2 m high × 1.6 m wide). Predicted particle collection efficiencies generally agreed with published data and ranged from less than 1% for 0.875-µm particles to approximately 32% for 15-µm particles.

Silicon Carbide (SiC) has the potential to significantly improve electronics. As a material, it can conduct heat better, carry larger currents and can give faster responses compared to today’s technologies. One way to produce SiC for use in electronics is by growing a thin layer in a CVD-reactor (chemical vapour deposition). A CVD-reactor leads a carrier gas with small parts of active gas into a heated chamber (susceptor). The gas is then rapidly heated to high temperatures and chemical reactions occur. These new chemical substances can then deposit on the substrate surface and grow a SiC layer. This thesis investigates the effect of different opening angles on a susceptor inlet in a SiC horizontal hot-walled CVD-reactor at Linköping University. The susceptor inlet affects both the flow and heat transfer and therefore has an impact on the conditions over the substrate. A fast temperature rise in the gas as close to the substrate as possible is desired. Even temperaturegradients vertically over the substrate and laminar flow is desired. The CVD-reactor is modeled with conjugate heat transfer using CFD simulations for three different angles of the inlet. The results show that the opening angle mainly affects the temperature gradient over the substrate and that a wider opening angle will cause a greater gradient. The opening angle will have little effect on the temperature of the satellite and substrate.